This invention relates to an improved process for producing decorated glass-ceramic articles in which an article of crystallizable glass which is decorated with ceramic color is converted into a glass-ceramic article by firing with simultaneous burning-in of the ceramic color.
Glass-ceramic articles, for example, stove burner covers, are decorated for aesthetic or also technical reasons, for example to mark cooking zones. For this purpose, a ceramic color is applied to the article by methods known per se, for example screen printing or the transfer image technique. The ceramic colors are glass-like, chemically fairly resistant, thin coating compositions, also called enamel colors, which are applied in a finely dispersed form, for example as a powder or suspension, to the article to be decorated and are fused to the surface to which it is applied by a heat treatment (burning-in) to give a color layer which firmly adheres to the thus decorated article. To save thermal energy, the procedure generally employed in the decoration of glass-ceramic articles is that the unfired glass article is decorated and the burning-in and the ceramicizing of the article is subsequently carried out simultaneously in one working. Such a process has been described, for example, in German Patent Specifications 3,505,922 and 3,600,109. The glass used for such a glass-ceramic article is, for example, a crystallizable glass from the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 system. Such glasses have a composition range, described for example, in U.S. Pat. Nos. 3,788,865 and 4,192,688, in % by weight, of 50-75 SiO.sub.2, 16-35 Al.sub.2 O.sub.3, 3-5.5 Li.sub.2 O and variable quantities of nucleating agents, the quantity of Li.sub.2 O+nucleating agent being at least 5.5% by weight. The nucleating agents used are 3-7 wt. % of TiO.sub.2, 1.5-3 wt. % of ZrO.sub.2 or 2-5 wt. % of P.sub.2 O.sub.5. Minor quantities of alkaline earth metal oxides may also be present.
The ceramicizing of these glasses, i.e., the conversion of the glass composition into a glass ceramic, takes place under the conditions well known to those skilled in the art. Usually the glass is treated, as described, for example, in U.S. Pat. No. 4,192,688, for at least 15 minutes at a temperature which is 15.degree. to 140.degree. C. above the upper annealing point (glass viscosity=10.sup.14 dP) for nucleation and then caused to crystallize at a temperature at least 190.degree. C. above the upper annealing point for up to 10 hours. For optimized results, the precise temperature/time values must be experimentally determined for the particular glass ceramic, which determination is well within the scope of the normal capacity of a person of average skill in the art. The decoration applied to the glass body is burnt-in during this temperature treatment.
The burnt-in decoration color forms a layer on the glass-ceramic article. When decorated glass-ceramic plates are used as stove burner covers, the decorated layer cannot be made in any desired thickness, since the closest contact possible between the glass-ceramic plate and the bottom of the pan is to be provided. The effect of this limitation on the layer thickness is that it is no longer possible to use any desired colors for the decoration, especially those colors which have only a relatively weak hiding power and would therefore require a greater thickness. Because the layer thickness of the decoration colors is limited, frequent use of the stove burner plate cover result in the decoration being attacked by the chafing by the bottom of pans thereon or even abraded by it, since the pan bottom above all chafes to and fro on the raised surface of the decoration.
It is an object of the invention to provide a process for producing decorated glass-ceramic articles, in which the decoration can also be applied in a greater thickness. It is another object to provide such a process in which the surface of the decoration is in substantially the same plane as the undecorated portions of the glass-ceramic article. Other objects will be apparent to those skilled in the art to which this invention pertains.